1. Field of the Invention
This invention relates to a method of detecting secondary electron charge on the inner surface of the neck of a cathode ray tube. The method of the invention is particularly useful during manufacture of the cathode ray tube.
2. Description of the Prior Art
In a display device, such as a television or monitor, which uses a cathode ray tube (CRT), electronic charge accumulates on the inner surface of the side walls of the tube around the neck during the course of normal use of the CRT. This charge, known as secondary electron charge, originates from the electron beams within the CRT, and generates an electric field which subsequently influences the electron beams. This can cause problems particularly, but not exclusively, with the convergence of the beams in a 3 electron gun type CRT e.g. for a colour TV or RGB monitor.
It is necessary to adjust the display device to compensate for the secondary electron charge. The convergence is usually adjusted by altering the position of a ring magnet provided on the neck of the CRT. However, when the CRT has just been manufactured there is no secondary electron charge on the neck, so if the convergence is adjusted in that condition, mis-convergence, also called jumping convergence or convergence drift, will occur later when the device is in use and secondary electron charge has been generated.
It has been proposed to generate secondary electron charge during the ageing process which the CRT undergoes following fabrication so that the convergence can then be adjusted under conditions similar to those of the device in use to avoid the mis-convergence problem. The secondary electron charge that accumulates on the inner surface of the neck of the CRT persists for at least one year even with the CRT not in use, therefore the secondary electron charge needs to be generated only once and then the convergence adjustments made will be correct for most future use of the CRT.
The above mentioned ageing process is done on a partially assembled display device including a CRT mounted on a pallet on a conveyor of a production line. The ageing process typically consists of subjecting the CRT to a temperature of 40.degree. C. for 2 hours with the electron beams switched on, but just displaying a white field. The device is then allowed to cool before the convergence is adjusted. It has been found experimentally that sufficient secondary electron charge is generated for the mis-convergence to be compensated for if the following condition is satisfied: EQU B.times.t.gtoreq.6000 (1)
where B is the screen brightness in cd/m.sup.2 and t is the ageing time in minutes.
A problem can arise during the ageing process because of the device entering the stand-by mode, also known as the protection mode or power saving mode. This might occur as a result of a poor electrical connection between the device on the pallet and the pallet on the conveyor, especially when the pallet is transferred from one conveyor to another, in the ageing process, or may be the result of the initial parameter settings of the device. If the device is in the stand-by mode for some or all of the ageing process, then insufficient secondary electron charge will be accumulated on the inner surface of the neck of the CRT since the above condition (1) will not have been fulfilled. The convergence will not then be correctly adjusted causing mis-convergence in later operation of the device.
In the above procedure there is no way of proving that the secondary electron charge had been generated, as it is not practical to measure the secondary electron charge directly.